In a variety of lasers an annular cylindrical configuration of the lasing medium is used. This has pronounced advantages especially in high power lasers (chemically, electrically or optically pumped). The annular configuration makes possible a compact design, efficient gas cooling, gas renewal, a pumping uniformity, etc. The annular configuration has the drawback of complicating the generation of single low order transverse mode operation where a compact beam region is required where the mode originates on the axis. By feedback, the mode in the compact region controls the radiation in the annular region.
One way of obtaining a central beam region where a low order single transverse mode may be attained is to use an additional intracavity element of the axicon or waxicon type which transforms the annular beam into a compact cylindrical one, or vice versa. Unstable resonators, in which there are used axicons or waxicons as intracavity elements, are well known. Such unstable resonators are very sensitive to optomechanical instabilities and to misalignments of optical elements. This sensitivity is a serious problem, especially with industrial lasers which are operated under severe environmental conditions. Hitherto one of the means of overcoming this sensitivity was to replace the flat feed-back mirror by a corner retroreflector, which reduces the optical alignment sensitivity by one or two orders of magnitude. With industrial lasers of the CO.sub.2 type, which have a large volume and a high power output, the use of such a corner retroflector may be expensive. For a relatively short-gain medium unstable resonators are not well suited. The low output coupling coefficient which is required causes two problems:
(i) Degraded transverse mode discrimination, and thus a loss of some of the advantages of unstable resonators;
(ii) Generation of a thin annular output beam which is a disadvantage for industrial applications.